OA2 (actually all the O** series I believe, like OD3, etc.) are cold-cathode voltage regulator tubes. Generally filled with mixes of neon/argon/xenon. They glow pretty colors, which I think is why they are used. Generally they conduct at a certain voltage level and can be run in series for higher voltage regulation. (e.g. OA2 @ 105v x 3 in series = 315v regulated.). They are not really in the audio signal path at all, so tube rolling them probably won't affect sound at all.
Enjoy the glow,
-Ed
Enjoy the glow,
-Ed